We don't know who decided that vampires ought to have long hair, but that's how you see them represented in fiction more often than not. Joe Manganiello famously played one on-screen in 'True Blood,' but has kept the style going after the series ended.
If anything, Manganiello's hair is even longer now than it was back then. It's fuller, wilder, and trails down to his chin. He's even grown a companion beard to go with his hair, and it suits him perfectly.